This invention relates to a process for isomerizing 1,4-dichloro-2-butene to yield 3,4-dichloro-1-butene or 3,4-dichloro-1-butene to yield 1,4-dichloro-2-butene, characterized in that the catalyst used comprises a compound of the formula CpFe(CO)2X.
3,4-Dichloro-1-butene is an important intermediate in the production of 2-chloroprene, which is used on a large industrial scale as a monomer in the production of polychloroprene rubber.
When butadiene is chlorinated, a mixture of cis-1,4-dichloro-2-butene, trans-1,4-dichloro-2-butene and 3,4-dichloro-1-butene is obtained which contains approx. 65% cis- and trans-1,4-dichloro-2-butene and approx. 35% 3,4-dichloro-1-butene. These isomers are usually present in the mixture in equilibrium, wherein the ratio is determined by production conditions. For simplicity""s sake, cis- and trans-1,4-dichloro-2-butene are hereinafter referred to together as 1,4-dichloro-2-butene. This mixture may be separated by distillation on the basis of differing boiling points (1,4-dichloro-2-butene: 154-9xc2x0 C. and 3,4-dichloro-1-butene: 123xc2x0 C.). Since only 3,4-dichloro-1-butene is suitable for the production of 2-chloroprene, the 1,4-dichloro-2-butene must be isomerized to yield 3,4-dichloro-1-butene.
Conventional processes for isomerizing 1,4-dichloro-2-butene to yield 3,4-dichloro-1-butene or vice versa are based upon the use of suitable isomerization catalysts, which ensure that equilibrium is rapidly established between the isomers in 1,4-dichloro2-butene or 3,4-dichloro1-butene at elevated temperatures. In most processes, metal salts of copper are used in the presence of further additives, which serve to achieve elevated reaction rates.
DE-A-2 138 790 discloses a process for isomerizing 1,4-dichloro-2-butene to yield 3,4-dichloro-1-butene or vice versa at 80 to 160xc2x0 C. by means of copper naphthenate, dinitrile and amide. DE-A-2 143 157 describes an isomerization process in the presence of copper salts and oxime derivatives at 80 to 160xc2x0 C. DE-A-2 200 780 claims a process which contains a mixture of a copper compound and an organic phosphorus compound as catalyst. DE-A-2 107 468 discloses the use of copper naphthenate and nitro compounds, while DE-A-2 130 488 discloses the use of copper naphthenate and nitroanilines. DE-A-2 212 235 describes an isomerization process by means of a copper compound and urea derivative. DE-A-2 206 971 claims the use of a mixture of copper compound and an aniline derivative containing chlorine. U.S. Pat. No.4,895,993 describes an isomerization process in the presence of a catalyst consisting of a copper compound and a dithiocarbamate or trithiocarbamate derivative.
Rostovshchikova et al. describes in Zh. Obschch. Khim. 1994, 64, 12, the use of triphenylphosphine or in Kinet. Katal. 1992, 33, 314 the use of various dialkyl sulfides in the presence of copper halides for catalytic isomerization. In Arm. Khim. Zh. 1987, 40, 709, Asatryan. et al. describes the action of various isomerization catalysts based on halide salts of copper, iron or zinc in the presence of amine derivatives such as triethylamine, diethylamine, triethanolamine, ethylenediamine or aniline. Asatryan et al. investigated in Arm. Khim. Zh. 1988, 41, 278 the action of macrocyclic polyethers or polyethylene glycols and in Arm. Khim. Zh. 1988, 41, 273 the influence of benzonitrile, nitrobenzene, DMF, dimethyl sulfone or acetophenone.
A disadvantage of all these processes is that the transformation rates are comparatively low and a large quantity of unwanted secondary products is formed. Moreover, elevated concentrations of the particular catalysts are required in order to achieve economically necessary isomerization rates, which entails considerable effort in recovering the catalyst and gives rise to large quantities of waste containing heavy metals. The described systems are, furthermore, extremely corrosive and require special materials if it is to be possible to perform the isomerization on an industrial scale.
In J. Organomet. Chem. 1971, 29, 307-311, Henrici-Olivxc3xa9 and Olivxc3xa9 describe catalysts for isomerizing dichlorobutenes, among which cyclopentadienyliron dicarbonyl dimer, [CpFe(CO)2]2, wherein Cp denotes cyclopentadienyl, has proved to be a highly active catalyst, which may be used without the addition of activity-promoting additives. The disadvantage of the described catalyst is its elevated price, which does not justify use on an industrial scale.
The object of the present invention was accordingly to provide a catalyst system which first, ensures elevated transformation rates, catalyzes selectively, and with reduced formation of secondary products, may be used at low concentration and gives rise to less corrosion.
This object is achieved by the provision of a process for isomerizing 1,4-dichloro-2-butene to yield 3,4-dichloro-1-butene or 3,4-dichloro-1-butene to yield 1,4-dichloro-2-butene, characterized in that the catalyst comprises a compound of the formula CpFe(CO)2X, wherein Cp denotes a cyclopentadienyl derivative of the general formula (I), 
wherein
R1 to R5 mutually independently denote H, C1 to C12 alkyl, C5 to C8 cycloalkyl, which may, in turn, bear C1 to C12 alkyl groups, C6 to C14 aryl, alkylaryl, arylalkyl, wherein two adjacent residues may together form saturated or unsaturated C3 to C14 cycles, or denotexe2x80x94SiR6R7R8, wherein R6 to R7 may mutually independently mean C1 to C4 alkyl, C5 to C8 cycloalkyl or C6 to C14 aryl, and X denotes F, Cl, Br, I.
The invention relates to a process for isomerizing 1,4-dichloro-2-butene to yield 3,4-dichloro-1-butene or 3,4-dichloro-1-butene to yield 1,4-dichloro-2-butene, characterized in that the catalyst comprises a compound of the formula CpFe(CO)2X, wherein Cp denotes a cyclopentadienyl derivative of the general formula (I), 
wherein
R1 to R5 mutually independently denote H, C1 to C12 alkyl, C5 to C8 cycloalkyl, which may, in turn, bear C1 to C12 alkyl groups, C6 to C14 aryl, alkylaryl, arylalkyl, wherein two adjacent residues may together form saturated or unsaturated C3 to C14 cycles, or denotexe2x80x94SiR6R7R8, wherein R6 to R7 may mutually independently mean C1 to C4 alkyl, C5 to C8 cycloalkyl or C6 to C14 aryl, and X denotes F, Cl, Br, I.
C1-C12 alkyl are taken to mean all linear or branched, saturated or unsaturated alkyl residues having 1 to 12 C atoms known to the person skilled in the art, such as methyl, ethyl, n-propyl, i-propyl, n-butyl, i-butyl, t-butyl, n-pentyl, i-pentyl, neopentyl, n-hexyl, i-hexyl, heptyl, octyl, nonyl, decyl, undecyl and dodecyl, together with the unsaturated homologues thereof.
C5 to C8 cycloalkyl are taken to mean all cyclic alkyl residues having 5 to 8 C atoms known to the person skilled in the art, such as cyclopentyl, cyclohexyl, cycloheptyl, cyclooctyl, together with the unsaturated homologues thereof.
C6 to C14 aryl are taken to mean all aryl residues having 6 to 14 C atoms known to the person skilled in the art, such as phenyl, naphthenyl, fluorenyl, anthracenyl and phenanthranyl.
Preferred cyclopentadiene derivatives are cyclopentadienyl, methylcyclo-pentadienyl, indenyl, tetrahydroindenyl and fluorenyl. The process is advantageously performed by
a) adding the iron complex according to the present invention to 1,4-dichloro-2-butene or 3,4-dichloro-1-butene at a temperature in the range from 40 to 200xc2x0 C., preferably in the range from 100 to 150xc2x0 C.,
b) allowing the catalyst to act for between 1 and 180 minutes, preferably between 15 and 45 minutes, in order to establish an equilibrium,
c) continuously removing a mixture of 1,4-dichloro-2-butene and 3,4-dichloro-1-butene and then separating it by distillation,
d) introducing the unwanted component from the distillation performed in c) into the reaction system and optionally
e) simultaneously with c), continuously supplying 1,4-dichloro-2-butene and/or 3,4-dichloro-1-butene to the reaction system.
Additionally, two or more different iron complexes according to the present invention may also be used in the form of a mixture.
The process may proceed either discontinuously or continuously at between 0.01 bar and 10 bar, preferably between 0.1 and 1.0 bar, wherein it is advisable initially, to produce a relatively highly concentrated solution of the catalyst system, preferably 10xe2x88x921 to 1 mol of Fe/L, in 1,4-dichloro-2-butene or 3,4-dichloro-1-butene and to add continuously thereto, respectively, a relatively large quantity of 1,4-dichloro-2-butene or 3,4-dichloro-1-butene, such that the desired concentration of the catalyst is obtained, wherein respectively 1,4-dichloro-2-butene or 3,4-dichloro-1-butene is continuously supplied, a mixture of 1,4-dichloro-2-butene and 3,4-dichloro-1-butene is continuously removed and then separated by distillation.